Revert "M2: regulators: fixup: make RPM_VREG_ID_PM8921_S2 regulator sleep_selectable"

 * This is causing high power drain and instability.

This reverts commit 298d95f32f9d28f42fab9444e5bcc2db5b0ccb2b.

Change-Id: Ia4ee3ff562fa0bd93dd7699eb74a60c89a9603bd